Suggest multiple tokens when multiple remotes are configured (or consolidating on a single instance)

[stefanvanburen]

Feature

In the case that someone has configured their buf.gen.yaml with multiple remotes, e.g.:

version: v2
# ...
plugins:
  - remote: buf.build/protocolbuffers/go
    out: gen/go
  - remote: buf.example.com/example/example
    out: gen/go

... it would be nice to suggest to them that they may want to configure multiple tokens. In this instance, configuring a single token for buf.build will result in token failures for buf.example.com, and configuring a single token for buf.example.com will either result in token failures (for private buf.build modules), or eventually hitting buf.build's rate limits on generation, which is confusing when a token is technically provided.

Additionally/alternatively, we could consider pointing out that generally users don't want to use multiple remotes (since plugins are typically synced to other BSR instances), and may want to consider consolidating on a single instance.

We could say something like:

$ buf generate
Failure: the BUF_TOKEN environment variable is set, but is not valid. Set BUF_TOKEN to a valid Buf API key, or unset it.
NOTE: You have multiple plugin remotes configured (`buf.build` and `buf.example.com`), you may either want to configure a token for each remote <link-to-docs / example>, or consolidate on a single remote.